Abstract During the electromigration of lithium ions from molten LiClKCl mixtures into FeS three of the phases formed are Li 2FeS 2 (X phase), Li 3Fe 2S 4 (Z phase), and LiK 6Fe 24S 26Cl (J phase). These phases and the copper-substituted X phase (Li 1.33Cu 0.67FeS 2) were prepared by high-temperature solid-state reactions between the respective metal sulfides, and their electrical conductivities were measured over temperatures ranging from 20 to 550°C. The results show that the X and copper-substituted X phases are semiconductors with conductivities and activation energies in the range of 0.1 to 10 (ohm-cm) −1 and of 8 to 21 kJ mole −1, respectively. The Z phase has a conductivity orders of magnitude less, and the J phase decomposes. A change in the slope of the Arrhenius plot occurs at 225°C, probably associated with some residue of the phase change in the FeS component. The electronic structures determined with photoelectron spectroscopy reveal the presence of two valence states for iron in the X, copper-substituted X, and Z phases. The valence bands consist of overlaping Fe(3 d), Cu(3 d), and S(3 p) electronic densities.